DESCRIPTION: Causalgia, or Complex Regional Pain syndrome Type II (CRPSII), is a form of pain that can develop in the extremities following damage to a peripheral nerve. Ongoing pain is accompanied by mechanical and thermal hyperalgesia. The etiology of CRPSII involves long-term alterations in the function of the peripheral and central nervous systems. Unfortunately, once these changes occur, the symptoms they produce are recalcitrant to treatment. Thus, the most effective treatment of CRPSII would be one that opposes the peripheral or central nervous system changes and that could be applied before these changes develop. The mechanisms by which high and low frequency electrical peripheral nerve stimulation (HFS and LFS, respectively) are believed to alleviate pain suggest that these therapeutic modalities directly oppose the central nervous system changes that develop with CRPSII. However, because people only seek medical care after symptoms develop, it remains unknown if electrical peripheral nerve stimulation can prevent any of the symptoms of CRPSII. Our preliminary investigation using a rat model of CRPSII-like mononeuropathy indicates that HFS, when delivered daily for one hour with the first treatment occurring immediately after surgery, can block the development of thermal hyperalgesia. The objective of the present study is to define the most effective, clinically realistic stimulation parameters for preventing the development of mechanical and thermal hyperalgesia in the same rat model. Because not everyone develops CRPSII after a nerve injury, the question of when HFS must be administered relative to the development of symptoms is important. The first specific aim of the study is to determine when in the course of symptom development HFS must be applied in order to attenuate thermal hyperalgesia. The second aim is to determine if increasing the duration of treatment with HFS can alter the development of mechanical allodynia and enhance the attenuation of thermal hyperalgesia. Because the means by which LFS is believed to produce analgesia is distinct from that of HFS, the third aim of the study is to determine the effectiveness of LFS in blocking or attenuating the development of mechanical and thermal hyperalgesia.